Since the semiconductor fabrication apparatus includes a series of stages for introducing, processing and carrying semiconductor substrates, a number of sensors are built in the apparatus. Exemplary sensors include a sensor for detecting the presence or absence of a substrate, a sensor for confirming the position or attitude of the substrate, and sensors for detecting edge, notch, orientation flat, thickness, shift, tilt and other parameters. In this application, photoelectric sensors capable of detecting the presence or absence of an object or any change of surface state by utilizing light are mostly used.
The photoelectric sensor is composed of an emitter section for emitting input light and a receiver section for receiving output light. If emitted light is blocked or reflected by an object to be detected, the quantity of light reaching the receiver section changes. The receiver section detects this change, converts it into an electric signal, and delivers the output. The light sources used in the sensors are mainly those of visible light, typically red, or green and blue for color discrimination, and infrared light.
Many semiconductor devices use single crystal silicon substrates while germanium, gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), gallium nitride (GaN), silicon carbide (SiC), and the like are also used. Since all these substrates are opaque, the sensors in the semiconductor apparatus are designed for opaque substrates.
For the detection of substrates in the semiconductor apparatus, front and back surfaces of substrates are given suitable means for enhancing the certainty of detection. For example, in the preparation of silicon substrates, if both surfaces of wafers are mirror-like surfaces, the front and back surfaces are not discriminatable. Then, the sensors in the processing apparatus do not perform well, or mirror-surface wafers slip away as they are carried forward. If the front and back surfaces of a substrate are separately processed, then the sensors in the semiconductor apparatus perform well because of a quality difference between the front and back surfaces. See Patent Document 1.
Recently, there arises a need to fabricate devices on transparent substrates rather than opaque substrates. Inter alia, synthetic quartz glass is substantially free of impurities and has excellent properties including light transmittance, heat resistance, coefficient of thermal expansion, and insulation. The glass is expected to find use in many fields, for example, as semiconductor devices, typically high temperature polysilicon thin film transistors (TFT), and micro-electro-mechanical systems (MEMS).